1. Field of Invention
Embodiments described generally relate to composite structures and shaped articles comprising thermoplastic vulcanizates and a method of preparing thermoplastic vulcanizates particularly suitable therefore.
2. Description of Related Art
Thermoplastic vulcanizate are well known subset of a group of polymeric materials call thermoplastic elastomers (TPE). TPE materials are both elastomeric and thermoplastic, yet are distinguished from thermoset rubbers, which are elastomeric but not thermoplastic due to the cross-linking or vulcanization of the rubber, and are distinguished from general thermoplastics which are generally stiff and hard, but not elastomeric. Thermoplastic vulcanizates are a class of TPE where cross-linked rubber forms a dispersed, particulate, elastomeric phase within a thermoplastic phase of a stiff thermoplastic such that TPE properties are achieved. The thermoplastic vulcanizates, TPVs or TPV compositions, are conventionally produced by dynamic vulcanization. Dynamic vulcanization is a process whereby a rubber component is crosslinked, or vulcanized, under intensive shear and mixing conditions within a blend of at least one non-vulcanizing thermoplastic polymer component while at or above the melting point of that thermoplastic. Typically, then the rubber forms cross-linked, elastomeric particles dispersed uniformly in the thermoplastic. See, for example U.S. patents U.S. Pat. Nos. 4,130,535, 4,311,268, 4,594,390 and 6,147,160. Dynamically vulcanized thermoplastic elastomers (TPVs) consequently have a combination of both thermoplastic and elastic properties.
Conventional plastic processing equipment can extrude, inject, or otherwise mold, and thus press and shape TPV compositions into useful products alone or in composite structures with other materials. Thus these TPV compositions are used in fabricated parts made by using standard polymer processing techniques like extrusion, co-extrusion, injection molding, co-injection molding, rotational molding, blow molding, co-blow molding, calendaring and thermoforming. These fabricated parts comprising thermoplastic vulcanizates can be made light in weight and attractive, with good durability, and can be reprocessed at the end of their product life to produce a new product. For these reasons, thermoplastic vulcanizates are widely used in industry.
Given this wide use many different polymeric modifiers have been added to provide different properties to basic TPV compositions. For example, EP969043 discloses a TPV composition that has improved elongation to break and toughness, where the TPV comprises A) from 20 to 75 parts by weight of rubber which has been dynamically vulcanized in the presence of a rubber curative; B) from 25 to 80 parts by weight of a combination of 40 to 80 parts by weight of semi-crystalline polypropylene having a melting temperature of at least 120° C. and 60 to 20 parts by weight of a polypropylene copolymer having a peak melting temperature equal to or above 25 and less than 120° C.; wherein said semi-crystalline polypropylene has a heat of fusion of more than 80 joules/gram, and said polypropylene copolymer has a heat of fusion of less than 75 joules/gram and comprises 80 to 95 weight percent repeat units from polypropylene and 5 to 20 weight percent repeat units from one or more other unsaturated olefin monomers having 2 or from 4 to 12 carbon atoms, based on the weight of said polypropylene copolymer; and wherein said rubber is selected from an ethylene-propylene-diene rubber, natural rubber, butyl rubber, halobutyl rubber, halogenated rubber copolymer of p-alkylstyrene and at least one C4-, isomono-olefin, a copolymer of isobutylene and divinyl benzene, a rubber homopolymer of a C4-8 conjugated diene, or a rubber copolymer having at least 50 weight percent repeat units from at least one C4-8 conjugated diene, or combinations thereof. A wide range of proportions is covered and no particular blending sequence is favored, but addition of the copolymer is exemplified as being added to the prepared or formed basic TPV compositions.
The presence of the dynamically vulcanized, cross-linked rubber particles in TPVs, however, can make these TPVs difficult to process. As a result, the presence of flow marks on injection molded articles surface and the presence of surface imperfections on the extruded profiles like hard spots, die lines, die deposit and screen pack blocking during extrusion which also commonly called die plugging. These surface imperfections of final articles are even more visible with soft TPVs which contain more rubber ingredient and consequently the most demanded products in different applications due to their higher elastic property. These surface imperfections are often make existing TPVs unfit in many applications, particularly when the surface of finished articles is visible like extruded weather seals used in automotive and construction applications, different articles made by injection molding like soft touch, sealing profiles like glass encapsulation made by over molding, grips, mats, over-molded skin for automotive interiors, etc. The visible welding lines in injection molding applications are an important weakness of existing TPVs which often results also to a lower weld lines strength. Another area of high interest in the market that existing TPVs have an issue to penetrate is in hose/tube applications for potable water where a smooth interior surface is sought to avoid the build up inside of hose/tube that can lead to microbiological activity.
Conventional attempts to improve the processability of thermoplastic vulcanizates have resulted in the sacrifice of mechanical properties of the TPV such as compression set, e.g., when different rubber is selected or different polymeric modifiers are added, or heavy blooming of additives when such are used as process improvers. There is a need, therefore, for TPVs that have improved processability while retaining good elastomeric mechanical properties.